[RN] Move unifiedSyncLane back to dynamic (#28430)

This surfaced a bug because it wasn't on everywhere, moving back to
dynamic while we investigate the bug

packages/shared/forks/ReactFeatureFlags.native-fb-dynamic.js

@@ -26,6 +26,7 @@ export const enableDeferRootSchedulingToMicrotask = __VARIANT__;
 export const enableUseRefAccessWarning = __VARIANT__;
 export const passChildrenWhenCloningPersistedNodes = __VARIANT__;
 export const useMicrotasksForSchedulingInFabric = __VARIANT__;
+export const enableUnifiedSyncLane = __VARIANT__;
 
 // Flow magic to verify the exports of this file match the original version.
 ((((null: any): ExportsType): DynamicFlagsType): ExportsType);

packages/shared/forks/ReactFeatureFlags.native-fb.js

@@ -24,6 +24,7 @@ export const {
   enableUseRefAccessWarning,
   passChildrenWhenCloningPersistedNodes,
   useMicrotasksForSchedulingInFabric,
+  enableUnifiedSyncLane,
 } = dynamicFlags;
 
 // The rest of the flags are static for better dead code elimination.
@@ -51,7 +52,6 @@ export const disableInputAttributeSyncing = false;
 export const disableIEWorkarounds = true;
 export const replayFailedUnitOfWorkWithInvokeGuardedCallback = __DEV__;
 export const enableScopeAPI = false;
-export const enableUnifiedSyncLane = true;
 export const enableCreateEventHandleAPI = false;
 export const enableSuspenseCallback = false;
 export const disableLegacyContext = false;

scripts/flow/xplat.js

@@ -14,4 +14,5 @@ declare module 'ReactNativeInternalFeatureFlags' {
   declare export var enableUseRefAccessWarning: boolean;
   declare export var passChildrenWhenCloningPersistedNodes: boolean;
   declare export var useMicrotasksForSchedulingInFabric: boolean;
+  declare export var enableUnifiedSyncLane: boolean;
 }